The present invention concerns an identification system, and in particular a code reader, for reading one- and/or two-dimensional codes that are arranged at different distances. The code reader employs a position resolving light receiver mounted in a housing in which associated imaging optics is arranged.
Known identification systems project an image of a code onto an image plane via an imaging optics. A position resolving light receiver is located in the image plane and has multiple, light-receiving pixels arranged linearly or in matrix form.
The differing light contrasts of the code are reproduced by the projected image and cause varying photocurrents in the individual light-receiving pixels of the light receiver which can be used to generate signals which identify the content of the code.
Such coding systems have many applications. For example, they can be used to identify and/or control individual objects in transportation systems. The present invention is not limited to processing particular codes and applies to all types of information carriers which can be photoprocessed.
Efficient coding systems are expected to contain increasing amounts of information on coding surfaces that are as small as possible. This requires that the identification system have a high spatial resolution. Particularly high demands are placed on the coding system when the positional location of the code can vary over a relatively large distance range. Efficient coding systems must further exhibit a high degree of readability, which requires, amongst others, efficient projection optics. These two requirements, that is, the ability to generate high quality images of objects that can be spaced apart over a wide range, and at the same time provide efficient projection optics, demand that the coding system be equipped with an automatic focusing system.
Known identification systems with automatic focusing systems distinguish between:                1) pure distance adjustments for different object spacings by adjusting the so-called “back focal distance”, which is measured from the vertex of the last back surface of a lens to its focal point, and        2) a change in the focal length of the imaging optics (a zoom characteristic) for changing the image scale for different object spacings.        
For adjusting the distance, the entire imaging optics can be moved along the optical axis, for example, while the position resolving light receiver remains stationary. For such applications, it is typical to employ a servo motor which engages an appropriate mechanism at the imaging optics so that activation of the servo motor converts the rotary motions of the latter into linear movements of the optics. Light-efficient receiving objects with long focal lengths have a relatively large mass so that focusing motions require substantial amounts of energy. Further, the bearings of such systems are subjected to mechanical wear and tear, which can lead to a deterioration of the components requiring repair and/or replacement. Such an approach to focusing is further relatively slow and typically requires relatively much more time so that such systems cannot adequately react when the object distances change rapidly.
In another known system, the distance adjustment is accomplished by stationarily mounting the imaging optics and moving the position resolving light receiver, including, when applicable, the entire circuit board on which the optics might be mounted with all its electronic components. In such an arrangement, the light receiver as well as other electric components mounted on the board are subjected to significant mechanical stresses. In addition, such arrangements require multi-path electrical connections which are subject to continuous motions and resulting wear and tear. Such installations, when used over the life cycle of an identification system, may be subject to between 107 to 109 operating cycles or more. Interruptions of the electrical connection to and from the light receiver and the other components on the board and/or their mechanical wear and tear are of course highly undesirable.
WO 93/14470 discloses a mobile optical code reader in which the back focal distance, that is, the effective distance between the imaging optics and the position resolving light receiver, is adjustable for focusing. WO 93/14470 accomplishes this by giving the light beam between the imaging optics and the position resolving light receiver a z-shaped path. An arrangement for folding the light beam in this manner is a planar reflecting mirror connected to a pivotable drive quadrant. An advantage of such a position adjustment system is that both the imaging optics and the position resolving light receiver can be stationarily fixed in a housing. However, these advantages are offset by the very complicated and costly nature of pivotally jointed drive quadrants. The reason for this is that in such systems the connector which mounts the deflecting mirror must have both ends rotatably connected to rods of the quadrant. In addition, it is necessary that the two rods be rotatably connected to the instrument or housing base. Thus, the connector and the two rods require four separate rotary joints. Especially for instances when the imaging optics has a relatively short focal length, precise focusing requires movements over very small distances in the range of a few μm which must be precise and precisely reproducible. To make this possible, the rotary joints must be tight and free of play and tolerances over a large number of operating cycles. This significantly enhances costs and requires more space.
U.S. Pat. No. 6,801,260 discloses an arrangement in which a movable mirror is arranged between fixed projecting optics and a fixed light receiver. The mirror is moved either linearly or pivotally to change the distance to the projecting optics. This changes the back focal length so that the images on the position resolving light receiver of objects with different object distances are always sharp and clear.
Such a distance adaptation for different object spacings has the disadvantage that for more distant objects relatively smaller image sizes are reproduced on the picture sensor. As a result, in such situations greater object distances reduce the ability for an error-free recognition of smaller objects.
To avoid this, such identification systems, in addition to adjusting the distance setting, require that the image reproduction scale, that is, the focal length of the imaging optics, be adjusted to reflect the actual distance of the object.